Generally speaking, a voltage controlled oscillator (abbreviated VCO) is a voltage controlled element, mainly for adjusting outputted oscillation frequency by controlling the input voltage. For enhancing overall performance, further improvement on the circuit of the voltage controlled oscillator is carried out by many manufacturers.
A voltage controlled oscillator, proposed in Taiwan utility model patent no. M407549, for example, includes a resonance unit, a coupling unit, an output unit and an adjustment unit. The resonance unit is provided with a first terminal, a second terminal, a third terminal receiving a bias voltage, and a fourth terminal receiving a first control voltage, as well as generating a first signal at resonance frequency and a second signal at resonance frequency, respectively, in accordance with the bias voltage and the first control voltage. The coupling unit comprises a first transistor and a second transistor. The first transistor is provided with a first terminal electrically connected to the first terminal of the resonance unit, a second terminal, a third terminal, and a control terminal electrically connected to the second terminal of the resonance unit, the third terminal receiving a voltage signal sufficient for reducing the critical voltage of the first transistor. The second transistor is provided with a first terminal electrically connected to the second terminal of the resonance unit and the control terminal of the first transistor, respectively, a second terminal, a third terminal receiving a voltage signal with non-zero magnitude of voltage, and a control terminal electrically connected to the first terminal of the resonance unit and the first terminal of the first transistor, respectively. The output unit is provided with a first terminal electrically connected to the first terminal of the resonance unit, a second terminal electrically connected to the second terminal of the resonance unit, a third terminal, and a fourth terminal, as well as the output unit is allowed for receiving the signal at resonance frequency whereby an output resonance signal is outputted via the third terminal and the fourth terminal thereof. The adjustment unit comprises a fifth capacitor and a sixth capacitor, the fifth capacitor being electrically connected at two terminals thereof to the first terminal and the second terminal of the first transistor, respectively, the sixth capacitor being electrically connected at two terminals thereof to the first terminal and the second terminal of the second transistor, respectively.
In the above prior art, the adjustment unit is composed of the third capacitor, the fourth capacitor, the fifth capacitor, the sixth capacitor, the third inductor and the fourth inductor. Input impedance looking in the direction away from the resonance unit from the first terminal of the second transistor is not featured by negative capacitance, such that the voltage controlled oscillator is only applicable to the operation in low-frequency band with limited adjustable frequency range.